For many years, gold has been a commercially important material, not only because of its esthetic properties, but also because of its electrical, physical and chemical properties. For example, it is electrically conductive and highly resistant to corrosion. These properties have led to increasing gold use in modern high technology applications such as micro-electronic circuitry and electrical contacts. Due to its relative scarcity and resulting high price, gold is generally used as a thin layer on another material. The layer can be applied by a plating process.
The large increase in the price of gold in recent years has stimulated intensive efforts to increase the efficiency of gold plating processes. These efforts have had many aspects. For example, effort has been directed toward reducing the thickness of the gold platings while retaining the desirable characteristics of the thicker platings. Further effort has been directed toward development of gold plating systems that plate selectively and thus do not waste gold by plating in undesired areas. Effort has also been directed toward reducing the volume of the plating solution and, therefore, the amount of gold within the plating system. The latter effort has been intimately connected with the development of high speed gold plating systems. Although there is no precise dividing line between high speed and low speed gold plating, slow speed is typically considered to be between 10 and 20 mA/cm.sup.2, and high speed is typically considered to be between 50 and 400 mA/cm.sup.2.
High-speed gold plating presents several difficulties and problems that were not present in the prior art low-speed plating methods. Effort has therefore been directed toward development of new apparatus and methods that overcome these difficulties and problems.
One problem that is present in all gold plating baths, but is especially acute with high speed baths, arises because the composition of the plating bath changes continually with use. The change in bath composition makes it difficult to produce a series of platings having uniform properties because of the changes in plating chemistry.
Although many factors contribute to the compositional change in plating baths, the change in the solution composition in high-speed gold plating baths results primarily from three factors. First, there is solution drag-out and the subsequent addition of water. Drag-out is the term used by those working in the art to represent the gold removed from the plating bath by the plated pieces as they are removed from the bath. Second, the concentration of potassium ions varies because potassium cyanoaurate, KAu(CN).sub.2, is conventionally added to replenish the gold. Potassium cyanoaurate is sometimes referred to as potassium gold cyanide. Third, an acid is generally added to control the pH. The last step is required because use of potassium cyanoaurate as the replenishing salt leads to an increase in pH. Potassium cyanide reacts with water to form HCN, a weak acid, and potassium hydroxide. Some of the HCN, boiling point 26 degrees C, escapes, and it is therefore necessary to adjust the pH by adding an acid. Typical acids include citric and phosphoric. However, the potassium ions do not leave the plating bath, and the potassium ion concentration will increase.
The increase in potassium ion concentration is an especially undesirable change in plating bath composition because the concentration may eventually reach a point where the concentration of potassium salts saturates, and they begin to crystallize out of the solution. Undesirable effects result. The crystallization interferes with the normal solution flow, and also alters the solution composition and the properties of the plated material. For example, at 30 degrees C, a typical gold concentration of 30 gms/liter may be maintained up to a K+ concentration of only 3 M.
Methods exist for controlling the potassium ion concentration and maintaining it below saturation. For example, the potassium ion concentration may be controlled and kept below saturation by increasing the drag-out ratio. For any given drag-out ratio, the potassium ion concentration will eventually reach an equilibrium concentration after some number of bath turnovers. A turnover is defined as completed when the gold initially present is plated, this amount is replenished, and the solution volume adjusted. The equilibrium concentration depends upon the drag-out ratio used, and higher drag-out ratios lead to lower potassium ion concentration. Thus, crystallization of the potassium salt may be prevented, but only at the price of unattractively high drag-out ratios. For example, drag-out ratios as high as 12 percent might have to be used. This is undesirable because it necessitates expensive systems for recovering gold from the rinse water.